Coating doctor apparatus

ABSTRACT

In coating doctor apparatus in which an air doctor is used to remove coating from a freshly coated web trained about a backing roll, internal baffles are eliminated and the contours of the coating return chamber are designed using flow control principles so that strong vortices and attendant misting are avoided in the coating return chamber, while at the same time subambient pressures generated along the curved surfaces of the chamber cause strong entrainment of air through the spaces between the edges of the coating chamber and the backing roll and air doctor and prevent coating material from escaping from the coating system.

United states Patent 1191 Reba et al. 1 May 8, 1973 [54] COATING DOCTORAPPARATUS 3,397,673 8/1968 Mahoney et a1 ..118/63 3,445,270 5/1969 Lorck..118/63 x Inventors: Imants Reba, Vancouver, Wash.; Edward C.Wolthausen, Oaklawn,

Ill.

Assignee: The Black Clawson Company,

Hamilton, Ohio Filed: Jan. 24, 1972 Appl. No.: 220,192

Related U.S. Application Data [63] Continuation of Ser. No. 28,741,April 15, 1970,

abandoned.

[52] US. Cl ..118/63 [51] Int. Cl. ..B05c 11/06 [58] Field of Search..118/63, 21;

[56] References Cited UNITED STATES PATENTS 3,078,496 2/1963 Doran etal. ..15 306 A x Primary Examiner-John P. McIntosh Attorney-Marechal,Biebel, French & Bugg [57] ABSTRACT In coating doctor apparatus in whichan air doctor is used to remove coating from a freshly coated webtrained about a backing roll, internal baffles are eliminated and thecontours of the coating return chamber are designed using flow controlprinciples so that strong vortices and attendant misting are avoided inthe coating return chamber, while at the same time subambient pressuresgenerated along the curved surfaces of the chamber cause strongentrainment of air through the spaces between the edges of the coatingchamber and the backing roll and air doctor and prevent coating materialfrom escaping from the coating system.

11 Claims, 1 Drawing Figure PATENTEUHAY' 8 91a INVENTORS IMANTS REBA 8|EDWARD C. WOLTHAUSEN BY W A TTOR/VEYS COATING DOCTOR APPARATUS Thisapplication is a continuation of Ser. No. 28,741, filed Apr. 15, 1970,now abandoned.

BACKGROUND OF THE INVENTION Coating systems which utilize an air knifeto doctor excess coating from a continuous web have found wide spreaduse in the industry as a result of the many advantages offered by thistype of doctoring system over those using doctor rods, scrapers and thelike. Despite the many advantages of the air knife type of doctoringsystem, one well recognized disadvantage of such systems is thedifficulty in preventing the doctored coating from escaping from thecoating system. This not only constitutes a health hazard but may alsocause malfunctioning of the coater components and other machinery in thearea by settling out on them.

Various remedies have been proposed as a solution to this problem.Generally they have taken the form of enclosing the doctoring area witha coating return chamber with the edges of the coating return chamberpositioned as closely as possible to the air doctor and the backing rollto provide as air tight an enclosure as possible. Additionally, bafflesare used within the coating return chamber to deflect coating removed bythe air doctor down into the chamber, from whence it is withdrawn,usually through the use of vacuum pumps or the like.

It will be apparent, however, that there is a practical limit to theextent to which the edges of the coating return chamber and the bafflesmay be placed in proximity to the backing roll and the air doctor blade.Additionally, it has been found that by placing the baffles very closeto the backing roll, which they must to be effective, a secondaryatomization of the coating is caused, resulting in a mist which coatsthe air doctors underside and also tends to escape through the gapbetween the air doctor and the return chamber wall.

It has also been found that existing coating return chamber geometryinduces strong vortex flows which cause misting, foaming, precipitation,and drying of the coating material, and the intensity of these vortexflows within the coating return chamber actually increases withincreasing baffle effectiveness.

SUMMARY OF THE INVENTION A coating doctoring system according to thepresent invention eliminates the necessity of baffles or the like andthe positioning of both edges of the coating return chamber in closeproximity to the backing roll and air doctor. In fact, one of the upperedges of the coating return chamber is purposely spaced from the backingroll since the design of the coating return chamber, based upon flowcontrol principles, results in a strong entrainment of air between theedges of the coating return chamber and the backing roll which preventsthe escape of coating mist from the system.

At the same time, the elimination of baffles in the coating returnchamber and the particular shape of the return chamber itself minimizemisting caused by secondary atomization and the generation of vorticeswithin the chamber.

Additionally, the shape of one of the chamber walls is such that thecoating removed tends to attach to this wall, from which it is collectedfor recirculation.

It will also be seen that atomizing nozzles may be utilized to spray afine mist of water into the coating chamber to precipitate coatingparticles and permit their ready removal from the system along with theliquid coating attached to one wall of the coating return chamber.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE shows a coatingdoctoring system according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As seenin the drawing, thecoating doctoring system includes a backing roll 10, about which istrained a freshly coated web 11, the latter being carried by the backingroll 10 in the direction indicated by the arrows past an air doctor 12.The air doctor has an outlet 13 through which is ejected a high velocityjet of air to remove excess coating carried by the web 11, and means,not shown, may be provided for varying the position of the air doctorfrom its solid line position to that shown in dotted lines in thedrawing.

Positioned beneath the air doctor and backing roll is a coating returnchamber, the main components of which are a flow control wall 20 and ashroud 21. The flowcontrol wall 20 includes a convexly curved inlet end22 and a convexly curved throat section 23 joined to the inlet sectionby a planar section 24, while the shroud 21 consists of a curved entryend 26 and a substantially planar throat section 27. Additionally, boththe flow control wall and shroud may be provided with concavely curvedterminal portions 25 and 28, respectively, which, by gently turning theair flow through the coating return chamber, conserve space but stillmaintain a uniform air stream through the system.

The flow control wall and shroud are provided with side plates, as at 30and 31, and end walls, one of which is shown at 32, extend between theflow control wall and shroud to complete the enclosure. It will also beseen that portions of side plates 31 extend upwardly to enclose aportion of the web and backing roll to prevent air flow into the systemfrom the sides. Additionally, wall 20 may be provided with a hinge 33 topermit the upper part of the wall to be pivoted downwardly for readyaccess to the interior of the coating return chamber for cleaning,maintenance, and the like.

It will be noted that the planar section 24 forms, with the opposedsurface of the backing roll 10, a narrow slot, on the order ofapproximately inch, through which the air stream passes as a highvelocity jet. It will also be noted that the radius of curvature ofsection 23 is appreciably greater than that of the backing roll 10. In aphysical phenomenon known as the Coanda Effect a tendency exists for ahigh velocity flow from a narrow slot to attach itself to a curvedsurface. Further observation indicates that where this flow is between apair of curved surfaces, the flow will attach to the surface having thegreatest radius of curvature. Thus, by creating a high velocity jet bymeans of the planar section 24 and positioning the curved section 23with a radius of curvature appreciably greater than that of backing roll10 immediately downstream of the section 24, coating carried by the airflow from the air doctor will tend to attach itself to the flow controlwall surface rather than reattach itself to the curved surface of theweb trained about the backing roll upstream of the point of coatingremoval.

it will be apparent from the drawing that the inlet end of the shroud 21is spaced an appreciable distance from the backing roll and in aninstallation this distance was set at approximately 4 inches. This iscontrary to conventional coating return chamber construction in that inconventional construction both walls of the coating return chamber arepositioned as closely as possible to the backing roll and air doctorblade to prevent the escape of coating mist. In the present system,however, the geometry of the coating return chamber is such as to createa strong entrainment of air between both the flow control wall and thebottom of the air doctor blade 12 and the backing roll 10 and the entryend of the shroud and the backing roll.

As noted above, conventional coating return chamber design in which thewalls of the chamber are positioned in close proximity to the backingroll and air doctor blade tends to create vortices within the coatingreturn chamber which in turn generate coating mist. in the coatingreturn chamber of the present invention, however, it will be seen thatthe flow control wall 20 and shroud 21 extend downwardly away from thebacking roll in gently diverging relationship. It will also be notedthat the inlet end of the shroud is spaced from the backing roll adistance at least equal to and preferably greater than the minimumspacing between the deflection wall 20 and the shroud 21. These twodesign features result in a smooth flow of air through the two inletsinto the coating return chamber adjacent the inlet end 22 of the flowcontrol wall and the entry end 26 of the shroud, thence through thethroat section of the coating return chamber and outwardly through thediffuser section 40.

Diffuser section includes diverging upper and lower walls 41 and 42,side walls, one of which is shown at 43, and flanges 44, which aresecured to complementary flanges on the terminal portions of the flowcontrol wall, shroud and side walls 32. Vanes 45 extend transversely ofthe diffuser section and are provided with threaded studs, not shown,which extend through slots 46 in the walls 43 and are secured therein bynuts,- also not shown, to permit some vertical adjustment of the vanes.

The diffuser section performs several important functions in the coatingdoctor system. Thus, since the cross sectional area of the diffuserincreases in the downstream direction the velocity of the fluid flow atthis point is appreciably, but not abruptly, reduced. Additionally, theuse of vanes in the diffuser gives a more uniform velocity profile atthe exit end thereof. As a result, if for some reason a back pressure isinadvertently created at the diffuser exit it will have a decreasedeffect on upstream air flow.

Although the elimination of baffles and the avoidance of strong vorticeswithin the chamber prevent the generation of coating mist by either ofthese two factors, there will nonetheless be some coating mist generatedby the impingement of the air jet upon the coated web, particularly whenoperating at high speeds. While most of the mist so generated combineswith the rejected liquid coating at the narrow slot formed by the flowcontrol wall and the backing roll, some mist is entrained in the airflow. Reduction of this mist may be accomplished by spraying moistureinto the chamber. Thus, atomizing nozzles 50 and 51 are positionedadjacent the inlet end 22 of the flow control wall and the entry end 26of the shroud. Nozzle 50 is supplied by means of a manifold 52 andnozzle 5! by means of a manifold 53, with both manifolds being connectedto a suitable source of liquid by means of conduits 54 and 55,respectively. Water issuing from the nozzles 50 and 51 emerges as anextremely fine spray, capturing any coating particles and then fallingto the wall 20 with the liquid coating removed from the coated web 11.

Immediately beneath the exit end of the diffuser a sump 60 having adownwardly depending drain 61 is positioned to receive liquid coatingattached to wall 42. If desired, a filter chamber 62 may be positionedin spaced relationship to the exit end of the diffuser to receiveexhaust air along with some ambient air.

It will thus be seen that the present invention, while eliminating theneed for baffles and positioning both edges of the coating returnchamber walls in extremely close proximity to the air doctor and backingroll, provides greatly improved removal of the doctored coating from thedoctoring area.

While the form of apparatus herein described constitutes a preferredembodiment of the invention, it is to be understood that the inventionis not limited to this precise form of apparatus, and that changes maybe made therein without departing from the scope of the invention whichis defined in the appended claims.

What is claimed is:

l. Coating apparatus comprising:

a. a backing roll adapted to carry a coated web in a downstreamdirection,

b an air doctor blade having portions including a bottom wall portiondefining an outlet positioned adjacent said backing roll to direct a jetof air in an upstream direction against a coated web carried in adownstream direction by said backing roll,

c. a flow control wall having an inlet end positioned adjacent saidbottom wall,

. said flow control wall extending from said bottom wall in an upstreamdirection to a point opposite and in closely spaced relationship to saidbacking roll,

. said flow control wall and said backing roll defining a narrow slot atsaid point,

f. said flow control wall extending from said slot downwardly and insaid upstream direction in a curve having a radius of curvature greaterthan the radius of said backing roll, and

. a shroud extending in spaced, opposed relationship to said flowcontrol wall and having an entry end positioned upstream of said inletend of said flow control wall,

. said entry end of said shroud being spaced from said backing roll adistance substantially greater than the spacing between said flowcontrol wall and said backing roll at said point.

. The apparatus of claim 1 wherein:

. said shroud and said flow control wall extending in downwardlydiverging relationship to each other.

3. The apparatus of claim 2 wherein:

a. the spacing between said entry end and said backing roll isapproximately equal to the minimum spacing between said shroud and saidflow control wall.

4. The apparatus of claim 2 wherein:

a. the spacing between said entry end and said backing roll is greaterthan the minimum spacing between said shroud and said flow control wall.

57 The apparatus of claim 1 wherein:

a. said flow control wall and shroud terminate in opposed terminalportions, and

b. a diffuser section is attached to and extends from said terminalportions c. said diffuser having wall portions extending in divergingrelationship to each other from terminal portions.

6. The apparatus of claim 5 further comprising:

a. vanes positioned in said diffuser section in spaced relationship tosaid diverging wall portions.

7. The apparatus of claim 6 wherein:

a. portions of said diffuser section define an exit end thereof, and

b. a sump is positioned beneath said exit end of said 10. The apparatusof claim 9 wherein said moisture injecting means comprises:

a. an atomizing nozzle positioned adjacent said inlet end,

b. an outlet of said nozzle being directed between said inlet end andsaid bottom wall.

11. The apparatus of claim 9 wherein said moisture injecting meanscomprises:

a. an atomizing nozzle positioned adjacent said entry end of saidshroud, b. an outlet of said nozzle being directed between said entryend and said backing roll.

1. Coating apparatus comprising: a. a backing roll adapted to carry acoated web in a downstream direction, b. an air doctor blade havingportions including a bottom wall portion defining an outlet positionedadjacent said backing roll to direct a jet of air in an upstreamdirection against a coated web carried in a downstream direction by saidbacking roll, c. a flow control wall having an inlet end positionedadjacent said bottom wall, d. said flow control wall extending from saidbottom wall in an upstream direction to a point opposite and in closelyspaced relationship to said backing roll, e. said flow control wall andsaid backing roll defining a narrow slot at said point, f. said flowcontrol wall extending from said slot downwardly and in said upstreamdirection in a curve having a radius of curvature greater than theradius of said backing roll, and g. a shroud extending in spaced,opposed relationship to said flow control wall and having an entry endpositioned upstream of said inlet end of said flow control wall, h. saidentry end of said shroud being spaced from said backing roll a distancesubstantially greater than the spacing between said flow control walland said backing roll at said point.
 2. The apparatus of claim 1wherein: a. said shroud and said flow control wall extending indownwardly diverging relationship to each other.
 3. The apparatus ofclaim 2 wherein: a. the spacing between said entry end and said backingroll is approximately equal to the minimum spacing between said shroudand said flow control wall.
 4. The apparatus of claim 2 wherein: a. thespacing between said entry end and said backing roll is greater than theminimum spacing between said shroud and said flow control wall.
 5. Theapparatus of claim 1 wherein: a. said flow control wall and shroudterminate in opposed terminal portions, and b. a diffuser section isattached to and extends from said terminal portions c. said diffuserhaving wall portions extending in diverging relationship to each otherfrom terminal portions.
 6. The apparatus of claim 5 further comprising:a. vanes positioned in said diffuser section in spaced relationship tosaid diverging wall portions.
 7. The apparatus of claim 6 wherein: a.portions of said diffuser section define an exit end thereof, and b. asump is positioned beneath said exit end of said diffuser to collectliquid coating therefrom.
 8. The apparatus of claim 7 furthercomprising: a. a filter chamber having an open end positioned in spacedrelationship to said diffuser section with said open end facing saidexit end, and b. a filter extending across said open end.
 9. Theapparatus of claim 1 further comprising: a. means for injecting moistureinto the area defined by said flow control wall and said shroud.
 10. Theapparatus of claim 9 wherein said moisture injecting means comprises: a.an atomizing nozzle positioned adjacent said inlet end, b. an outlet ofsaid nozzle being directed between said inlet end and said bottom wall.11. The apparatus of claim 9 wherein said moisture injecting meanscomprises: a. an atomizing nozzle positioned adjacent said entry end ofsaid shroud, b. an outlet of said nozzle being directed between saidentry eNd and said backing roll.